For years, scientists have known that rodents — and human babies — have different kinds of fat: White fat and brown fat. Brown fat burns calories; white fat just sits around. Now we know that adult humans have brown fat, too. And what if you could turn white fat into brown fat?
One kind of brown fat, according to the New York Times, only starts sucking up the white fat and burning it away when a person is cold. The other kind of brown fat — in mice, at least — appears during exercise. And it comes from white fat being turned brown.
Dr. Spiegelman suspects that humans, like mice, make brown fat from white fat when they exercise, because humans also have irisin in their blood. And human irisin is identical to mouse irisin.
"What I would guess is that this is likely to be the explanation for some of the effects of exercise," Dr. Spiegelman says. The calories burned during exercise exceed the number actually used to do the work of exercising. That may be an effect of some white fat cells turning brown.
Of course, that doesn't necessarily equal weight loss. The Times's Gina Kolata notes: "While exercise may induce brown fat in humans, it remains to be seen how important a source of calorie burning it is, researchers say." Which means that you could exercise a lot… but never get thinner. Kind of like Steven N. Blair, one of the nation's leading experts on the health benefits of exercise, who runs every single day and describes himself as "short and fat." But if you want to get a jump on the research, by all means, start exercising in a freezing room. Let us know what happens.